Spruce wood was pre-extracted, decayed with the white-rot fungus Phanerochaete chrysosporium to a 20% loss in lignin content, and then extracted successively with solvents of increasing polarity (pet. ether, chloroform, acetone, methanol, and aqueous dioxane). The methanol extract (0.5% of the decayed wood) was divided into acid and phenol fractions, and these further into high and low molecular weight components. The low molecular weight acid fraction was examined by high performance liquid chromatography and, after acetylation and methylation, by gas chromatography/mass spectrometry. The following compounds were identified: 4-hydroxybenzoic acid (I), vanillic acid (2), isovanillic acid (3), veratric acid (4), 4-hydroxy- 5-methoxyphthalic acid (7), m-hemipinic acid (8), dehydrodivanillic acid (9), and 2'-hydroxy-2,3'-dimethoxydiphenylether-4,5'- dicarboxylic acid (10), 4-Hydroxy-3-methoxymandelic acid (11) was also tentatively identified. Compounds 2, 3, 4, 8, and 9 were the most abundant. Vanillic acid (2), homovanillic acid, and acetovanillone were 4-O-methylated by ligninolytic cultures of Ph. chrysosporium, with methionine serving as methyl donor. Thus compounds 4, 6, and 8 (and possibly 3), containing 4-O-methyl groups, probably arose via fungal methylation. Results establish that lignin is degraded by white-rot fungi in part via low molecular weight aromatic acids, formation of which involves C alpha-C beta side chain cleavage with C alpha oxidation. (Refs. 32).